Logic state analyzers are used to monitor and record sequences of states that occur in a collection of digital signals in a system under test. A "state" is simply any one of the 2.sup.n logical patterns that n-many digital signals may experience. A sequence of addresses or a sequence of fetched instructions are examples of electrical activity describable as states in a microprocessing environment and that can be monitored by a logic state analyzer to record their "state flow."
To monitor the ongoing sequence of states in a system under test a logic state analyzer samples the electrical values of the signals of interest at times determined by one or more clock signals associated with the system under test. The sampled electrical values obtained are compared to thresholds of selected value and polarity to determine their logical values, each of which will be either true or false, one or zero. Each resulting collection of ones and zeros for a sample is a state in the ongoing sequence of states. It is also simply a binary value that may be stored in a memory. A series of such stored values is a record of the activity occurring in the system under test. Such a record may be termed a trace.
It can be useful for a trace to contain other information besides a simple list of states in the order they occurred. For example, it may be very useful to know the elapsed time between the states in the trace, or, perhaps the time between each state and an origin on a time axis. It can also be useful to know how many times some selected event occurred between each pair of consecutive states in the trace.
These additional data may be obtained by equipping a logic state analyzer with a counter connectable to a high speed clock signal for time measurements and connectable to a programmable state detection mechanism for event counts. Each time a state is stored as part of the trace the associated count is stored as well. The output form of the trace (display, printed table, etc.) includes the time or count information for each state in the trace.